I will add it is a d class amp and I think it is a joules thief / I want to try and repair it myself but need to identify the parts.One of the leads has a sign of being burnt.
if it is installed on the heat sink with some type of adhesive or goo i would think it could be some type of temperature sensor . so does your amp have a thermal protection , meaning if things get to hot will it shut down or go in protect mode